Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it. Printed circuit boards are used in all but the simplest electronic products. They are also used in some electrical products, such as passive switch boxes. Alternatives to PCBs include wire wrap and point-to-point construction , both once popular but now rarely used.
PCBs require additional design effort to lay out the circuit, but manufacturing and assembly can be automated. Specialized CAD software is available to do much of the work of layout. Mass-producing circuits with PCBs is cheaper and faster than with other wiring methods, as components are mounted and wired in one operation. Large numbers of PCBs can be fabricated at the same time, and the layout only has to be done once.
PCBs can also be made manually in small quantities, with reduced benefits. PCBs can be single-sided one copper layer , double-sided two copper layers on both sides of one substrate layer , or multi-layer outer and inner layers of copper, alternating with layers of substrate. Multi-layer PCBs allow for much higher component density, because circuit traces on the inner layers would otherwise take up surface space between components. The rise in popularity of multilayer PCBs with more than two, and especially with more than four, copper planes was concurrent with the adoption of surface mount technology.
However, multilayer PCBs make repair, analysis, and field modification of circuits much more difficult and usually impractical. Before the development of printed circuit boards electrical and electronic circuits were wired point-to-point on a chassis. Typically, the chassis was a sheet metal frame or pan, sometimes with a wooden bottom. Components were attached to the chassis, usually by insulators when the connecting point on the chassis was metal, and then their leads were connected directly or with jumper wires by soldering, or sometimes using crimp connectors, wire connector lugs on screw terminals, or other methods.
Circuits were large, bulky, heavy, and relatively fragile even discounting the breakable glass envelopes of the vacuum tubes that were often included in the circuits , and production was labor-intensive, so the products were expensive. Development of the methods used in modern printed circuit boards started early in the 20th century. In , a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. Thomas Edison experimented with chemical methods of plating conductors onto linen paper in Arthur Berry in patented a print-and-etch method in the UK, and in the United States Max Schoop obtained a patent  to flame-spray metal onto a board through a patterned mask.
Charles Ducas in patented a method of electroplating circuit patterns. The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in the UK around In a multi-layer printed circuit was used in German magnetic influence naval mines. Printed circuits did not become commonplace in consumer electronics until the mids, after the Auto-Sembly process was developed by the United States Army. Even as circuit boards became available, the point-to-point chassis construction method remained in common use in industry such as TV and hi-fi sets into at least the late s.
Printed circuit boards were introduced to reduce the size, weight, and cost of parts of the circuitry. In , a small consumer radio receiver might be built with all its circuitry on one circuit board, but a TV set would probably contain one or more circuit boards. The ECME could produce three radio boards per minute. During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity.
The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors , with ceramic disc capacitors and subminiature vacuum tubes soldered in place. Army, was assigned to Globe Union. Rubinstein the Cledo Brunetti Award for early key contributions to the development of printed components and conductors on a common insulating substrate. Rubinstein was honored in by his alma mater, the University of Wisconsin-Madison , for his innovations in the technology of printed electronic circuits and the fabrication of capacitors.
Originally, every electronic component had wire leads, and a PCB had holes drilled for each wire of each component. The component leads were then inserted through the holes and soldered to the copper PCB traces. This method of assembly is called through-hole construction. In , Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered.
The patent they obtained in was assigned to the U. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are inefficient since drilling holes is expensive and consumes drill bits and the protruding wires are cut off and discarded. From the s onward, small surface mount parts have been used increasingly instead of through-hole components; this has led to smaller boards for a given functionality and lower production costs, but with some additional difficulty in servicing faulty boards.
In the s the use of multilayer surface boards became more frequent. As a result, size was further minimized and both flexible and rigid PCBs were incorporated in different devices. As a result, components can be closer and the paths between them shorter. With multi-layer HDI PCBs the interconnection of stacked vias is even stronger, thus enhancing reliability in all conditions. The most common applications for HDI technology are computer and mobile phone components as well as medical equipment and military communication equipment. However, the cost is much lower. Recent advances in 3D printing have meant that there are several new techniques in PCB creation.
Manufacturers may not support component-level repair of printed circuit boards because of the relatively low cost to replace compared with the time and cost of troubleshooting to a component level. In board-level repair, the technician identifies the board PCA on which the fault resides and replaces it. This shift is economically efficient from a manufacturer's point of view but is also materially wasteful, as a circuit board with hundreds of good components may be discarded and replaced due to the failure of one minor and inexpensive part such as a resistor or capacitor.
This practice is a significant contributor to the problem of e-waste. A basic PCB consists of a flat sheet of insulating material and a layer of copper foil, laminated to the substrate.
Chemical etching divides the copper into separate conducting lines called tracks or circuit traces , pads for connections, vias to pass connections between layers of copper, and features such as solid conductive areas for EM shielding or other purposes. The tracks function as wires fixed in place, and are insulated from each other by air and the board substrate material.
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The surface of a PCB may have a coating that protects the copper from corrosion and reduces the chances of solder shorts between traces or undesired electrical contact with stray bare wires. For its function in helping to prevent solder shorts, the coating is called solder resist or solder mask. A printed circuit board can have multiple copper layers. A two-layer board has copper on both sides; multi layer boards sandwich additional copper layers between layers of insulating material.
Conductors on different layers are connected with vias , which are copper-plated holes that function as electrical tunnels through the insulating substrate. Through-hole component leads sometimes also effectively function as vias. After two-layer PCBs, the next step up is usually four-layer. Often two layers are dedicated as power supply and ground planes, and the other two are used for signal wiring between components.
A board may use both methods for mounting components. PCBs with only through-hole mounted components are now uncommon. Surface mounting is used for transistors, diodes, IC chips, resistors and capacitors. Through-hole mounting may be used for some large components such as electrolytic capacitors and connectors.
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The pattern to be etched into each copper layer of a PCB is called the "artwork". The etching is usually done using photoresist which is coated onto the PCB, then exposed to light projected in the pattern of the artwork. The resist material protects the copper from dissolution into the etching solution. The etched board is then cleaned. A PCB design can be mass-reproduced in a way similar to the way photographs can be mass-duplicated from film negatives using a photographic printer. In multi-layer boards, the layers of material are laminated together in an alternating sandwich: copper, substrate, copper, substrate, copper, etc.
Only the outer layers need be coated; the inner copper layers are protected by the adjacent substrate layers. FR-4 glass epoxy is the most common insulating substrate. Another substrate material is cotton paper impregnated with phenolic resin, often tan or brown. When a PCB has no components installed, it is less ambiguously called a printed wiring board PWB or etched wiring board. However, the term "printed wiring board" has fallen into disuse. In informal usage, the term "printed circuit board" most commonly means "printed circuit assembly" with components. The IPC preferred term for assembled boards is circuit card assembly CCA ,  and for assembled backplanes it is backplane assemblies.
For example, expansion card. A PCB may be "silkscreen" printed with a legend identifying the components, test points, or identifying text. Originally, an actual silkscreen printing process was used for this purpose, but today other, finer quality printing methods are usually used instead. Normally the screen printing is not significant to the function of the PCBA. A minimal PCB for a single component, used for prototyping, is called a breakout board.
The purpose of a breakout board is to "break out" the leads of a component on separate terminals so that manual connections to them can be made easily. Breakout boards are especially used for surface-mount components or any components with fine lead pitch. Advanced PCBs may contain components embedded in the substrate. The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides.
Horizontal installation of through-hole parts with two axial leads such as resistors, capacitors, and diodes is done by bending the leads 90 degrees in the same direction, inserting the part in the board often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength , soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave soldering machine. Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and it limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards, since the holes must pass through all layers to the opposite side.
Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB e. Through-hole devices mounted on the circuit board of a mids Commodore 64 home computer.
A box of drill bits used for making holes in printed circuit boards. While tungsten-carbide bits are very hard, they eventually wear out or break. Drilling is a considerable part of the cost of a through-hole printed circuit board. Surface-mount technology emerged in the s, gained momentum in the early s and became widely used by the mids. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes.
Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper.
However, prices of semiconductor surface mount devices SMDs are determined more by the chip itself than the package, with little price advantage over larger packages, and some wire-ended components, such as 1N small-signal switch diodes, are actually significantly cheaper than SMD equivalents. Each trace consists of a flat, narrow part of the copper foil that remains after etching. Its resistance , determined by its width, thickness, and length, must be sufficiently low for the current the conductor will carry.
Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in a planar form such as stripline or microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; conversely, they can be used as a deliberate part of the circuit design, as in distributed element filters , antennae , and fuses , obviating the need for additional discrete components.
The European Union bans the use of lead among other heavy metals in consumer items, a piece of legislature called the RoHS , for Restriction of Hazardous Substances, directive. PCBs to be sold in the EU must be RoHS-compliant, meaning that all manufacturing processes must not involve the use of lead, all solder used must be lead-free, and all components mounted on the board must be free of lead, mercury, cadmium, and other heavy metals. Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form an integral final piece of uniform thickness.
The size can be up to 4 by 8 feet 1.
Varying cloth weaves threads per inch or cm , cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation FR-4, CEM-1, G, etc.
There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit.
Thermal expansion is an important consideration especially with ball grid array BGA and naked die technologies, and glass fiber offers the best dimensional stability. FR-4 is by far the most common material used today. The board stock with unetched copper on it is called "copper-clad laminate". With decreasing size of board features and increasing frequencies, small nonhomogeneities like uneven distribution of fiberglass or other filler, thickness variations, and bubbles in the resin matrix, and the associated local variations in the dielectric constant, are gaining importance.
The circuitboard substrates are usually dielectric composite materials. Rigid flex circuits combine the best of both rigid boards and flexible circuits integrated together into one circuit. The two-in-one circuit is interconnected through plated thru holes. Rigid flex circuits provide higher component density and better quality control. Designs are rigid where extra support is needed and flexible around corners and areas requiring extra space.
While the design options are endless when combining rigid boards with flexible circuits, we have provided you with rigid flex combinations that are most commonly utilized.
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Connection Reliability — Connecting rigid layers with flexible cables is the foundation for combination rigid flex circuits. Lower Part Count — Compared to a traditional rigid board, combination rigid flex circuits require fewer parts and interconnections. Flexible Design Options — At Flexible Circuit Technologies, we pride ourselves on taking on the most complex of design challenges. Rigid flex circuits can be designed to meet highly complex and unimaginable configurations while utilizing a rigid substrate.
Rigid flex circuit designs could entail any of the following:. High Density Applications — More often than not, the rigid component of a rigid flex circuit is utilized for high density device population. In addition, flexible circuits allow for minutely narrow lines giving way to high density device population. Denser device populations and lighter conductors can be designed into a product, freeing space for additional product features. Package Size and Weight Reduction — Multiple systems in rigid boards create more weight and utilize more space.
Combining rigid boards with flexible circuits allows for a more streamlined design thus reducing package size and weight. Static Application — An application where flex circuits are needed only to install the circuit and fit it into its application also known as flex-to-fit or flex-to-install. In summary, if you have rigid flex design needs, Flexible Circuit can help.
Conductor options include:. Rigid-flex circuits are a hybrid construction flex circuit consisting of rigid and flexible substrates which are laminated together into a single structure. Rigid-flex circuits should not be confused with rigidized flex constructions are simply flex circuits to which a stiffener is attached to support the weight of the electronic components locally.
A rigidized or stiffened flex circuit can have one or more conductor layers. Thus while the two terms may sound similar, they represent products that are quite different. The layers of a rigid flex are also normally electrically interconnected by means of plated through holes. Over the years, rigid-flex circuits have enjoyed tremendous popularity among military product designer, however the technology has found increased use in commercial products. While often considered a specialty product for low volume applications because of the challenges, an impressive effort to use the technology was made by Compaq computer in the production of boards for a laptop computer in the s.
By , the use of rigid-flex circuits in consumer laptop computers is now common. Rigid-flex boards are normally multilayer structures; however, two metal layer constructions are sometimes used.
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Flexible Circuits are often used as connectors in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring. In LCD fabrication, glass is used as a substrate. If thin flexible plastic or metal foil is used as the substrate instead, the entire system can be flexible, as the film deposited on top of the substrate is usually very thin, on the order of a few micrometres.
Organic light-emitting diodes OLEDs are normally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display. Most flexible circuits are passive wiring structures that are used to interconnect electronic components such as integrated circuits, resistor, capacitors and the like, however some are used only for making interconnections between other electronic assemblies either directly or by means of connectors.
In the automotive field, flexible circuits are used in instrument panels, under-hood controls, circuits to be concealed within the headliner of the cabin, and in ABS systems. Consumer electronics devices make use of flexible circuits in cameras, personal entertainment devices, calculators, or exercise monitors. Flexible circuits are found in industrial and medical devices where many interconnections are required in a compact package. Cellular telephones are another widespread example of flexible circuits. Flexible solar cells have been developed for powering satellites.
These cells are lightweight, can be rolled up for launch, and are easily deployable, making them a good match for the application. They can also be sewn into backpacks or outerwear. Flexible Circuit technology has a surprisingly long history. Patents issued at the turn of the 20th century show clear evidence that early researchers were envisioning ways of making flat conductors sandwiched between layers of insulating material to layout electrical circuits to serve in early telephony switching applications.
One of the earliest descriptions of what could be called a flex circuit was unearthed by Dr Ken Gilleo and disclosed in an English patent by Albert Hansen in where Hansen described a construction consisting of flat metal conductors on paraffin coated paper. Curtis National Bureau of Standards Circular first issued 15 November the a brief discussion of creating circuits on what would have been flexible insulating materials e. An advertisement from the placed by Photocircuits Corporation in New York demonstrated their active interest in flexible circuits also. Today, flexible circuits which are also variously known around the world variously as flexible printed wiring, flex print, flexi circuits, are used many products.
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